Sensor fabric for shape perception

ABSTRACT

A sensor fabric has a plurality of flakes of a flexoelectric material and electrodes as threads for connecting the flakes together. Each flake has at least one pair of opposite edges connected by two separate electrodes. When the sensor fabric is placed in contact with the surface of an object, the flake is bent along the contour of the object surface. The bending of the flexoelectric flake generates a polarization voltage between its opposite edges, which can be measured via the electrodes to determine the local surface contour of the object.

Robotics, the engineering science and technology of robots, has come along way since the word “robot” was coined in the early twentiethcentury. Nevertheless, it is still a challenge to replicate or mimicsome basic human sensory functions in robotic devices. For instance, ithas been very difficult to mimic the sense of touch. While a human handcan easily sense the shape of an object and other attributes such astemperature and stiffness by simply touching the object, it has not beenan easy task for sensors in modern robotic devices to obtain shapeinformation by touch. Some prior attempts for shape perception by touchare based on MEMS devices for electro-mechanical responses. Such sensingdevices have many issues. They are bulky and complicated, and producesmall signals that require highly sensitive signal amplification fordetection. As a result, they are hardly useful for integration intosystems of interest, such as robotic hands.

BRIEF DESCRIPTION OF THE DRAWINGS

Some embodiments of the invention are described, by way of example, withrespect to the following figures:

FIG. 1 is a schematic top view of a sensor fabric in accordance with anembodiment of the invention;

FIG. 2 is a schematic top view of a rectangular flexoelectric flake andelectrodes in the sensing fabric of FIG. 1;

FIG. 3 is a schematic view of a hexagonal flexoelectric flake andelectrodes in accordance with another embodiment of the invention;

FIG. 4 is an enlarged view of a boron nitride flexoelectric sheet in abent form and creating a voltage between its two opposite edges;

FIG. 5 is a depiction of the boron nitride crystal lattice and a bendingdirection in relation to a reference lattice vector;

FIG. 6 is a plot showing the dependence of the polarization of thevoltage generated by a boron nitride flexoelectric sheet as a functionof the bending direction; and

FIG. 7 is a schematic depiction of a robotic hand using sensor fabricpatches on its finger tips for touch sensing.

DETAILED DESCRIPTION

FIG. 1 shows a sensor fabric 100 in accordance with an embodiment of theinvention. The sensor fabric 100 is formed of a collection of flakes 110of a flexoelectric material that are “threaded together” by electrodes112, 114, 116, and 118. As described in greater detail below, the sensorfabric can be used for touching sensing to detect the shape, texture,stiffness, and other attributes of an object, and can be made to have ahigh sensor density and high sensitivity.

The flexoelectric flakes 110 and the electrodes connecting them may besupported by a substrate 120 to provide structural strength anddimensional stability of the fabric 100. In some embodiments, thesubstrate 120 may be formed of a web material, such as a plastic sheet,that is flexible. The flexibility allows the sensor fabric to follow thesurface contour of an object when it is pressed against, or “touches,”the object surface. The flexoelectric flakes 110 and the electrodes maybe attached to or fabricated on the substrate 120 using known techniquesfor fabrication of semiconductor devices and MEMS devices.

FIG. 2 shows one flexoelectric flake 110 in the sensor fabric 100 andits connecting electrodes 112, 114, 116, and 118. As used herein, theterm “flexoelectricity” refers to an electromechanical property togenerate a voltage polarization in response to strain gradients. Theflake 110 is generally two-dimensional (2D) in that its thickness issignificantly smaller than its length and width. For example, the widthand length of the flake may range from tens of nanometers to micrometersor even larger, while the thickness of the flake in some embodiments maybe as small as one atomic layer. For sensing purposes, the flake 110 haselectrodes connected to at least one pair of its opposite edges. In theillustrated embodiment, the flake 110 has a rectangular shape (which maybe a square) and has two pairs of opposite edges. Electrodes 112, 118are connected to the opposite edges 130 and 132, respectively, whileelectrodes 116 and 114 are connected to the opposite edges 134 and 136,respectively. As described in greater detail below, having electricconnections to two or more pairs of opposite edges allows the detectionof the bending of the flake in different directions.

The operation of the sensor fabric 100 is based on the phenomenon thatwhen a flexoelectric flake 110 is bent, it produces a bias voltagebetween its opposite edges. The bias voltage depends on the direction ofthe bending with respect to the lattice axes of the flexoelectricmaterial forming the two-dimensional flake. Materials exhibiting suchflexoelectric property include two-dimensional non-centrosymmetricmaterials such as boron nitride (BN) and boron carbon nitride (BC₂N).

Contrary to three-dimensional (3D) solids of flexoelectric materials,two-dimensional (2D) sp²-bonded crystals are able to sustainsignificantly greater elastic structural distortions and exhibitexceptional forms of electromechanical coupling. It has also beendiscovered that 2D sp²-bonded non-centrosymmetric crystals like boronnitride or boron carbon nitride exhibit another unusual flexoelectriceffect: generating a macroscopic in-plane voltage polarization inresponse to out-of plane periodic atomic displacements. A boron nitride2D sheet is a piezoelectric dielectric due to a broken sub-latticeinversion symmetry. In the case of boron carbon nitride, there is afurther reduction in the sub-lattice symmetry, leading to the breakingof the three-fold point symmetry. In those 2D sheets, the induced localpolarization is independent of the sign of the curvature and quadraticin induced curvature. The net macroscopic polarization is allowed evenin cases of zero net strain, like in periodic deformation. Thepolarization can be quite large, on the order of 1 volt, which may beeasily detected by an external circuit without the need for sensitiveamplifying circuitry to amplify the signals.

For illustration of this phenomenon, FIG. 4 shows, on an atomic scale, asingle atomic layer sheet 160 of boron nitride (BN). The boron (B) atomsand nitrogen (N) atoms form a honeycomb crystal lattice with a hexagonalunit cell. When the sheet 160 is bent or curved into undulation alongthe line L, a bias voltage is generated between the two opposite edges162 and 164. This voltage can be connected by electrodes 166 and 168 toan external voltage detector 170 for measurement.

The voltage polarization generated by the BN sheet 160 depends on thebending amplitude and wavelength, as well as the direction of thebending wave with respect to the crystalline lattice. FIG. 5 shows thelattice structure 180 of boron nitride. The vectors a₁ and a₂ are thebasis vectors of the lattice. The vector λ indicates the periodicity andorientation of the bending wave. The angle θ represents the anglebetween the direction of the bending wave and a reference vector definedas (a₁+a₂), which is chosen in such a way that the shortest inter-atomicdistance in the lattice along this vector is from the boron atom to thenitrogen atom, along the line 182.

The angular dependence of the polarization P induced by the bending waveor corrugation is shown in FIG. 6. The polarization P varies as afunction of the angle θ, with the components of P changing as (−cos 2θ,sin 2θ). Detecting the voltages of two pairs of edges of the sheet indifferent directions, such as shown in FIG. 2, allows an analyzercircuit to determine the polarization and magnitude of the bendingexperienced by the sheet.

In FIG. 4, for illustration purposes, the BN sheet 160 is shown to havea single atomic layer. The polarization effect described above can beobserved in a thin BN film or flake with an odd number of layers, suchas 1, 3, 5, and on. Of the multiple layers, only one produces theuncompensated polarization, while the rest even number layers can beconsidered as a mechanically supporting structure or a scaffold.

Returning to FIG. 1, in accordance with a feature of an embodiment ofthe invention, the flakes are arranged in a two-dimensional matrix, andthe electrodes connecting the flakes are formed in a “crossbar”arrangement. For each edge pair (e.g., top/bottom or right/left), thereis a first group of generally parallel electrodes for connecting to thefirst edges of the flakes, and a second group of generally parallelelectrodes for connection to the second edges of the flakes. Theelectrodes in the second group extend at an angle to the electrodes inthe first group. For instance, in the illustrated embodiment, a firstgroup of electrodes 112 are connected to the right edges of the flakes,while a second group of electrodes 118 are connected to the left edgesof the flakes 110. In FIG. 1 the angle between the electrodes 112 and118 is illustrated to be about 90 degrees, but other angles may bechosen. Similarly, a third group of electrodes 116 are connected to thetop edges of the flakes 110, while a fourth group of electrodes 114 areconnected to the bottom edges of the flakes. In the illustratedembodiment, the electrodes 114 run parallel to the electrodes 112, andthe electrodes 116 run parallel to the electrodes 118. The crossbarstructure allows an edge pair of each flake to be individuallyaddressable for reading the bias voltage between the edges.

The flexoelectric flakes in the sensor fabric do not have to be squareor rectangular and can have other shapes. For example, FIG. 3 shows aflake 200 that is hexagonal. With the hexagonal shape, the flake 200 hasthree pairs of opposite edges, and each edge pair provides a componentof the voltage polarization when the flake is bent. The electrodesconnecting the flake edges can also be arranged into a crossbarstructure. In the illustrated embodiment of FIG. 3, the electrodes 202,204, and 206 are formed to be generally parallel and extend in a firstdirection, while the electrodes 212, 214, and 216 are generally parallelto each other and extend in a second direction at an angel from thefirst direction. The angle in the illustrated embodiment is about 60degrees. The opposite edges in each edge pair of the flake 200 areconnected, respectively, to one electrode running in the first directionand another electrode in the second direction.

The ability to detect the bending of individual flakes in the sensorfabric allows the sensor fabric to be used for touch sensingapplications. When the sensor fabric is pressed against the surface ofan object, each flake comes into contact with the object surface andconforms to the local surface contour. If the local surface contour hasa curvature or undulation, the flake is bent along the surface andgenerates bias voltages between its opposite edges. The bias voltagesbetween the edge pairs of the flake allow an analyzer to determine thecurvature and bending direction of the object surface in contact withthat flake. The aggregated information about the localized objectsurface contour as sensed by the flakes allows the analyzer to determinethe shape of the object.

Moreover, besides the object shape, other attributes of the objectsurface can also be detected using the sensor fabric. As one example,the stiffness of the object surface may be detected with the sensorfabric. This may be done by pressing the sensor fabric against theobject surface at different pressures. The yielding of the objectsurface in response to a pressure, which is an indication of the surfacestiffness, produces different surface contours under differentpressures. To measure the stiffness, the sensor fabric is first pressedagainst the surface with a first pressure, and the bias voltages of theflake edges are measured to detect the surface contour under thatpressure. The sensor fabric is then pressed against the surface under asecond pressure. The bias voltages of the flake edges are again measuredto determine the surface contour under the second pressure. The changesin the surface contour as measured by the flakes of the sensor fabric asa function of the pressure change may then be used to derive thestiffness of the object surface.

The temperature of the object surface can also be measured using thesensor fabric. To that end, in one embodiment as illustrated in FIG. 1,thermal couple wires 220 may be attached to selected points in thesensor fabric 100 for detecting the surface temperature of the object atdifferent locations.

As an example of the many possible uses of the sensor fabric, FIG. 7shows a robotic hand 250 that uses the sensor fabric for touch sensing.The robotic hand 250 has a plurality of fingers 251-255. Patches 256-260of the sensor fabric are attached to the tips the fingers. When therobotic hand 250 holds an object 266 with its fingers, the sensor fabricpatches 256-258 of the fingers 251-253 holding the object 266 arepressed against the object surface. The bias voltages between the edgesof the flexoelectric flakes in those sensor fabric patches 256-258 canbe measured to determine the local object surface shape at each patch.By moving the robotic fingers along the object surface, the overallshape of the object 250 can be mapped out. This mimics how a human handsenses the shape of an object. As mentioned earlier, other attributes ofthe object surface, such as its temperature and stiffness, can also bedetected.

It should be noted that the applications of the sensor fabric are notlimited to robotic devices. There are many other applications in whichthe sensor fabric can be used to provide shape perception or touchsensing that is not achievable with prior art sensors. For instance, thecompact form factor of the sensor fabric allows it to be used in placeswhere even MEMS devices are too bulky. Also, the fabric can beconstructed with chemically neutral materials to allow it to be used inharsh environments, including biological environments such as inside ahuman body as part of prosthetics, actuators, etc.

In the foregoing description, numerous details are set forth to providean understanding of the present invention. However, it will beunderstood by those skilled in the art that the present invention may bepracticed without these details. While the invention has been disclosedwith respect to a limited number of embodiments, those skilled in theart will appreciate numerous modifications and variations therefrom. Itis intended that the appended claims cover such modifications andvariations as fall within the true spirit and scope of the invention.

1. A sensor fabric for touch sensing, comprising: a plurality of flakes of a flexoelectric material, each flake having a first edge and a second edge opposite to the first edge, and opposing surfaces defining a plane, the first and second edges being disposed in the plane; and a plurality of electrodes connected to the flakes for sensing a bias voltage between the first and second edges of each flake, wherein the electrodes connected to the flakes include a first group of electrodes running in a first direction and a second group of electrodes running in a second direction, each electrode in the first group being connected to the first edges of the plurality of flakes, and each electrode in the second group being connected to the second edges of the plurality of flakes.
 2. A sensor fabric as in claim 1, wherein the flexoelectric material has a two-dimensional non-centrosymmetric lattice structure.
 3. A sensor fabric as in claim 2, wherein the flexoelectric material is boron nitride.
 4. A sensor fabric as in claim 2, wherein the flexoelectric material is boron carbon nitride.
 5. A sensor fabric as in claim 1, wherein each flake further includes a third edge and a fourth edge opposite to the third edge, and the electrodes further include a third group of electrodes running in the first direction and a fourth group of electrodes running in the second direction, each electrode in the third group being connected to the third edges of the plurality of flakes, and each electrode in the fourth group being connected to the fourth edges of the plurality of flakes.
 6. A sensor fabric as in claim 1, further including a flexible substrate supporting the flakes and electrodes.
 7. A sensor fabric as in claim 1, wherein the flakes have a rectangular shape.
 8. A sensor fabric as in claim 1, wherein the flakes have a hexagonal shape.
 9. A sensor fabric as in claim 1, wherein the first edge is substantially parallel to the second edge.
 10. A method of sensing a shape of an object, comprising: placing a sensor fabric in contact with a surface of the object, the sensor fabric has a plurality of flakes of a flexoelectric material and a plurality of electrodes connected to the flakes, each flake having a first edge and a second edge opposite to the first edge, and opposing surfaces defining a plane, the first and second edges being disposed in the plane; and sensing a bias voltage between the first and second edges of each flake caused by bending of the flake, wherein the electrodes connected to the flakes include a first group of electrodes running in a first direction and a second group of electrodes running in a second direction, each electrode in the first group being connected to the first edges of the plurality of flakes, and each electrode in the second group being connected to the second edges of the plurality of flakes.
 11. A method of sensing as in claim 10, wherein each flake of the sensor fabric further includes a third edge and a fourth edge opposite to the third edge, and the step of sensing further includes sensing a bias voltage between the third and fourth edges of each flake.
 12. A method of sensing as in claim 11, wherein the flexoelectric material has a two-dimensional non-centrosymmetric lattice structure.
 13. A method of sensing as in claim 12, wherein the flexoelectric material is boron nitride.
 14. A method of sensing as in claim 12, wherein the flexoelectric material is boron carbon nitride.
 15. A method of sensing as in claim 10, wherein the first edge is substantially parallel to the second edge.
 16. A method of detecting stiffness of an object, comprising: placing a sensor fabric in contact with a surface of the object, the sensor fabric having a plurality of flakes of a flexoelectric material and a plurality of electrodes connected to the flakes, each flake having a first edge and a second edge opposite to the first edge, and a third edge and a fourth edge opposite to the third edge, and opposing surfaces defining a plane, the first, second, third and fourth edges being disposed in the plane; pressing the sensor fabric against the object surface with a first pressure; determining a first contour of the object surface under the first pressure by sensing a bias voltage between the first and second edges and a bias voltage between the third and fourth edges of each flake caused by bending of the flake; pressing the sensor fabric against the object surface with a second pressure; and determining a second contour of the object surface under the second pressure, wherein differences between the first and second contours provide an indication of the stiffness of the object wherein the electrodes connected to the flakes include a first group of electrodes running in a first direction and a second group of electrodes running in a second direction, each electrode in the first group being connected to the first edges of the plurality of flakes, and each electrode in the second group being connected to the second edges of the plurality of flakes.
 17. A method of detecting stiffness as in claim 16, wherein the first edge is substantially parallel to the second edge. 